If the congregation Namaz is being taken place and all the front rows have been occupied and person comes later, should he stand in the next row from the right side, left side or the middle, or should he wait for someone else to come and join him?

Now a days objections are being made on the type of dresses that are being worn. Specially the shirts which the youngsters wear and come to offer prayers at the mosque which contain pictures of sportsmen and animals. Is it allowed to wear such dresses and offer prayers?